First featherweight!

[Mokichi]

Re: First featherweight!

Gimsons arrived a few days ago, and botbitz ESC's have been ordered.

In terms of battery I was looking at either Opti power LiPo or a cheaper (local) alternative

Otherwise two 9.6v NiMh packs from similar to this

I would prefer to voltage to be above the 14.4v of the Gimsons so I get a bit of extra speed/power. Also for either option could someone please recommend a good charger/balancer, thanks!

For a base plate I may use thinner HDPE (6mm) as there's less play room that I thought between the Gimson's plastic mounts and the 78mm wheels! I may purchase a couple of 100mm robochallenge wheels for this reason.

[maddox10]

Re: First featherweight!

Hannibalito 4 uses 170mm wheels on a set of 9.6V Gimsons. Speed is fine as you can see here, even if we only use a 3S LiPo pack.

[Mokichi]

Re: First featherweight!

I can imagine the speed would be fine if my calculations are correct! xD

17cm x pi = 51cm ish
51cm x 530 (RPM) = 27030cm/min
27030cm/min --> mph = just over 10mph

Because of the smaller wheels I'm using, either 78mm or perhaps the 100mm robochallenge ones, I'd need to use more voltage to get a more adequate speed. Again my calculations would suggest that without increasing the voltage.

78mm wheels = 5.2mph
100mm = 6.8mph

Probably the maths is wrong though xD

[maddox10]

Re: First featherweight!

RPM is a tad higher. 9.6V=530 rpm= 55.3 rpm/v so 11.1V gives us 613 rpm= 19.6km/h (12mph), and that nominal.

Full LiPo's are 4.2V, so add another mph when starting.

[RogueTwoRobots]

Re: First featherweight!

Also for either option could someone please recommend a good charger/balancer, thanks!

I've got two of these chargers:

http://www.giantshark.co.uk/power-a606- ... 05325.html

Can't really go wrong with them. They do everything from SLA through to various lithium types, come with a bunch of different charging leads, can be powered off the mains (a lot of chargers only have DC inputs, so require either big batteries or a separate AC power supply unit) and, best of all, they cost less than £30.

The downside for lipos etc is that you can only charge up to 6S packs as it doesn't have balance charge inputs for any larger packs. Depends on whether you envisage using larger packs in the future.

[typhoon_driver]

Re: First featherweight!

For a feather you wouldn't be using anything higher than 6s and if you are you would likely use two 5s packs for a 10s pack

[maddox10]

Re: First featherweight!

Because of the smaller wheels I'm using, either 78mm or perhaps the 100mm robochallenge ones, I'd need to use more voltage to get a more adequate speed. Again my calculations would suggest that without increasing the voltage.
78mm wheels = 5.2mph
100mm = 6.8mph

Probably the maths is wrong though xD

Unfortunatly,upping the voltage has another effect next to increasing the speed.

Basic principle, double the voltage, double RPM and double the amperes drawn.

RPM is per volt, and a constant for every type of permanent magnet brushed motors. The Gimsons 9.6V, after gearing do 55.3rpm/V

We also know that 30A is nice , and will allow motor and speedo to survive stall for a reasonable time- meaning, unless you're a utter fool and keep the power on for seconds even if Sir Killalot is parked on the feather- meaning 300W is our aim for these. Compared to the Speed 900 setups we use, it's absolutely anemic. But it does the job for a reasonable pricetag and ease of use.

Now, 300W ain't the max stall current in these motors. 9.6V*85A=800W+. Imagine what a 800W soldering iron can do.

But double the volts, you also double the amps flowing. Ergo 19.2*170A=3264W. Yes, but not as a motor, but as a fast melting heater. Magic smoke, thank you.

A rule of the tumb. Commercial motors -like Bosch 750, Iskra , wipermotors, window lifters, Maxon gearmotors and such can be overvolted safely for our purposes to 150%.
Batterydrills are a strange duck in the pound here. Some will cope, others will burn. The better the brand, the more will cope.

RC motors, if the brand says max XX volt, be assured it's the max, and even a tad over it.

But remember, an electric motor will only draw as much power as required to do the job. Dave's spinner proves that. During spin-up the amps go trough the roof, to taper off to very acceptable levels once up to speed.

Add this info to the fact that normal , and affordable, brushed motors are only 75-80% efficient, you can imagine that after a fight even non overvolted motors will get warm. Drawing 100W means 20-25W are converted into heat, and we solder a lot with a 20W soldering iron....

[PJ-27]

Re: First featherweight!

Here is a great page on the Biohazard website on motors. Also check out the other pages.

http://www.robotbooks.com/motor-tips.htm

Heat is proportional to current squared. So doubling the motor voltage will quadruple the heat!

[PJ-27]

Re: First featherweight!

Also the iMax B6 is a popular cheap charger. I think I got mine for £8 from Hong Kong! Although I believe it to be a copy as most cheap ones are.